The present invention is concerned with a novel class of materials that fits between the ordinary industrial laminates and research-type superlattices and nanolaminates, in regards to their properties and cost. The novel Interface-Defined nano-Laminated materials of this invention differ from both the large-scale laminates and the extremely fine-scale superlattices, due to their unique micro- and nano-structures produced by the novel methods of fabrication. In the new IDnL materials the interfaces between the alternative layers can be designed and fabricated from many different materials. Also, these interfaces can have unique properties and structures, which can be varied from nearly coherent to completely incoherent by varying the processing approach. The degree of deviation from perfect coherency at the interfaces potentially can be controlled without much increase in cost of the IDnL materials.
In general, laminates can be made with layers having a wide range of thickness. The terms laminated materials or laminates generally refer to materials that consist of many parallel layers of relatively thick (layer thickness, t>1 mm) dissimilar materials. Laminates are utilized in many diverse fields, such as food preparation (French and German pastry), penetration-resistant materials (armor, bullet-proof glass), heat shields for satellites (NASA, DOD), as well as tools (metal cutting inserts), and weapons (Japanese samurai swords)—just to name a few.
The properties of laminates, in general, are controlled by two factors i.e. the properties of the material within the layers and the properties of the interfaces between the layers. When the number of layers is small (in this case a material usually referred to as ‘layered’), it is predominately the properties of the materials within the individual layers that define the properties of the whole laminate. However, as the number of layers increases, the properties of the interfaces between the dissimilar layers begin to impose an ever increasing effect on the properties of the laminate. In some applications, it is the properties of the interfaces that are the determining factor in the performance of the whole laminate. For example, a reflecting insulator that consists of a number of metallic layers, each of which is an excellent conductor of heat and is separated from the next reflector by an air gap or vacuum, is, nevertheless, an excellent insulator because of the processes of reflecting and scattering of heat perpendicular to the metal/gas interfaces.
Laminates have many industrially useful properties. The properties of laminates are anisotropic, so they are often called ‘2-dimensional materials’, because their properties in the plane of the layers and perpendicular to that plane are drastically different. For example, heat conductivity in the crystal plane and perpendicular to the crystal plane of pyrolytic graphite can differ by three orders of magnitude; fracturing goes easily along the glass planes in laminated glass, but is quickly arrested in the direction perpendicular to the glass planes; electrical current propagates in planes, but not perpendicular to the planes in metal/oxide laminates utilized in super-capacitors, etc.
The anisotropic properties of laminates can be highly useful in impeding conduction of heat as well as propagation of fracture, or chemical attack. Regardless of the form of the propagating entity, laminate materials usually inhibit propagation of the energy or matter in the direction perpendicular to the layers, while dissipating this energy or matter along the surface of the interfaces.
From a conceptual point of view, the process of obstructing the energy propagation can be described by a similar mathematics in all these cases, be it heat, stress wave that causes fracturing, or diffusion—each interface constitutes a barrier that has to be overcome by the incoming energy or matter in order to proceed through the material. Though each barrier may be small, the sheer number of them and their sequential nature ultimately overwhelms the incoming energy or matter and slows down the rate of its flow through the material to a small fraction of the original value. To illustrate this point: consider, for example, that one barrier reflects or scatters only 0.01% of the incoming energy, or 0.0001, letting 0.9999 through. 100,000 of these barriers placed apart at a distance of 100 nm would attenuate the flow of incoming energy to 1% of the starting value after the distance of only 1 centimeter.
In most cases the scattering at an interface is much higher than 0.01%. For the example of heat scattering at a metal/gas interface, the scattering is controlled by the emissivity and reflectivity of the metal surface, which can be above 50%. This is why only a few reflectors are necessary to contain very high temperatures. However, even when the scattering coefficient is small, the sheer number of barriers gives tremendous power to the approach of laminated materials. This is one of the reasons why nano-laminates—laminates with thicknesses of individual layers of the order of 1 to 999 nm and preferably from 1-100 nanometers, and superlattices, which are a subset of nano-laminates with strong, clearly defined interfaces, have attracted so much interest in the last decades, both in research and industry.
In contrast to the laminates with macroscopic thick layers discussed above are the nano-laminated materials and superlattices that have been researched extensively since the late 1970s. These are extremely finely-layered materials with the thickness of individual layers of the order of 1 to 10 nm. They are also prohibitively expensive for industrial applications for reasons outlined below. The word ‘superlattice’ was coined by physicists, who were the early investigators of these materials, to emphasize the existence of extra peaks in X-ray diffraction patterns of these materials. Traditionally, the word ‘super-lattice’ is used with nano-layered materials with coherent interfaces, i.e., when the lattice planes are continuous from one phase to another across the interface. When the interfaces are incoherent, the material is usually referred to as ‘nano-layered’. (In the instant invention, the word ‘nano-laminate’ will be used for all these types of materials with layers of nanometer thickness up to 999 nanometers.) These nano-laminated materials have been found to have very intriguing and industrially-useful properties. The whole area is still an active research field in Materials Science and Physics. Electronic, magnetic, and mechanical properties of these materials are still actively researched, scientific conferences held, and new applications come out every year. New important properties, such as superior hardness/toughness combination, excellent wear resistance, super-modulus effects, superconductivity, optical waveguide properties, and magnetic properties are active areas of research in nano-laminates.
In the area of resistance to flow of heat, it was always expected that nano-laminates can be made to provide excellent resistance to heat flow on the basis of the multiple-barrier model discussed above. Each interface between dissimilar materials scatters phonons or electrons, which are the heat carriers in opaque solids. Recent experimental results [R. M. Costescu, et al, “Ultra-Low Thermal Conductivity in W/Al2O3 Nanolaminates, Science 303, 989 (2004)] with W/Al2O3 nano-laminates, having <60 pairs of layers, provides a confirmation of the above point of view, with thermal conductivity of k=0.6 W/mK measured. This result compares very well with thermal conduction of strongly disordered crystalline oxides that are in the range of 1 to 3, but significantly higher than powdered MgO (k=0.04) or silica-based carbon-added Aerogels (k=0.02).
Presently, despite their attractive properties for impeding energy flow, from the point of view of industrial and commercial applications, nano-laminates have some very serious drawbacks. That is, to manufacture these materials currently requires very expensive equipment, very clean conditions, and high vacuum, as the laminates are essentially built-up one atom at a time. To date, these materials have been fabricated utilizing magnetron sputtering or atomic layer deposition (ALD). Nano-laminates manufactured by these techniques usually have strongly-attached coherent interfaces, because of the perfection of the deposition and atomic uniformity of the interface. However, the size of samples made of these materials is limited, and the cost to make commercial products with these techniques is prohibitive with the state-of-the art techniques.
The desired degree of coherency at each interface depends on the application. As stated above, laminate materials usually inhibit propagation of the energy or matter in the direction perpendicular to the layers, while dissipating this energy or matter along the surface of the interfaces. Thus, to inhibit the propagation of energy, such as thermal energy or crack propagation perpendicular to the interfaces, it is desirable to have an incoherent interface between the layers of the laminate because coherent interfaces do not effectively scatter the energy perpendicular to them, alternatively, in many optical, electronic, and semiconductor-type applications, where electronic mobility or other transport properties must be optimized—highly coherent interfaces, without any dislocations, are a must.
Because of the high cost, nano-laminates are mainly used in high-tech-type industries, where the price of the product justifies the expense of making a material at the ‘breath-taking’ rate of 1 micron/hour. The fabrication methods currently used for making nano-laminates cannot be scaled-up to industrially meaningful dimensions because they are inherently prohibitively expensive.
Thus, a need exists for an industrially-scalable batch or continuous techniques to produce low-cost nanolaminates at a cost of at least an order of magnitude and preferably at least two orders of magnitude lower than is currently possible with the state-of-the-art techniques. In addition, a need also exists to be able to produce nanolaminates of much larger dimensions, such as higher areas. That is, a need exists for a process that is able to fabricate a low porosity nanolaminate material, in which each interface has a cross-sectional area of at least 0.1 square meter, preferably 1 square meter, and most preferably 10 square meters. The instant invention achieves the goal of providing an industrially-scalable methodology for fabricating large-area parts from nanolayered materials, which are already known in scientific research. Moreover, in the process of developing this methodology, these inventors have discovered a new class of nano-layered materials, termed IDnL, which cover the range of layer thickness between ordinary laminates and superlattices, as outlined above. These new materials have micro- and nanostructure which is very different from that of the two classes of laminated materials discussed above. Because these materials are fabricated from powders, which are eventually densified via rapid sintering, hot rolling, dynamic compaction, and such, the new materials have properties different from that of the already known laminated materials.
There are a lot of approaches, methods, and techniques that have been employed for making metal and ceramic laminates. The simplest approaches produce layers at least 100 microns in thickness and involve placing one layer on top of the other, which can be done by dipping in or painting wet slurries as well as by utilizing tapes. Other techniques that are able to deposit layer by layer, one after another, utilize chemical, physical, mechanical, explosive, or high-voltage approaches to deposit material on surfaces. Techniques that can produce micron-thick layers include ink-jet printing, silk-screen printing, plasma spraying, and the use of a Meyer bar or a Doctor blade. The thinnest nanometer-thick layers require the use of techniques, such as, chemical vapor deposition, physical vapor deposition, atomic layer deposition, electro-deposition, as well as magnetically and electrostatically-assisted sputtering in which layers are built-up one atom at a time. Other techniques, such as electrophoresis have been used to deposit ceramic nano-laminates from aqueous suspensions. All of the above nanometer layer techniques are expensive and inherently very slow not only because of the low rate of deposition but also because of the need to move the substrate between deposition stations or to change the precursor between layer depositions, as well as to allow the previous layer to dry or cure before the next layer can be applied. These techniques are more applicable to fabricating layered coatings. Considering the size of the required vacuum chambers and the cumbersomeness of each of these techniques, it would be prohibitively expensive to fabricate bulk parts with at least a square meter in area and 100,000 layers in thickness.
A few methods to make bulk nano-layered materials do exist, however. One such method is used in manufacturing exfoliated graphite, vermiculate, and mica-type thermal insulation. This method utilizes the natural property of these materials to form flakes. The individual flakes whose area varies from sub-micron to hundreds of millimeters are dispersed in a liquid. When the liquid is removed by evaporation, the flakes settle and form a nano-layered material. However, the individual layers in such structures are not continuous or uniform and the thickness cannot be easily controlled. In addition, it is impossible to make multi-component nano-layered materials, i.e. nano-laminates with adjacent layers having different composition or structure, with a nanometer layer thickness employing this technique.
A multiple extrusion step approach has been utilized in the electronics industry for more than 50 years to make nanometer thick layers in Channeltron photo-multiplier tubes. In this process, sacrificial glass rods coated with a different glass are bundled together in a hexagonal array and drawn down to a very small diameter through many drawing steps. After the sacrificial glass is removed, micron sized holes separated by nanometer thick walls formed by the coating remain. A similar process is currently used in superconductor wire processing to make fibers that consist of large number of closely packed cores. In this case, ceramic superconductor wires are assembled in a closely-packed bunch within a copper outer tube and then extruded to ever smaller diameter tubes to make thin wires that consist of thousands of thin electrically-isolated superconducting wires. These approaches are conceptually similar to the current invention, however these approach are directed towards making single layer 1-dimentional structures—tubes and wires—not 2-dimensional multi-layered bulk materials.
To summarize, there are no approaches that exist in the current state of the art for making large quantities of high surface area nano-laminates with thousands of continuous nanometer thick layers of different metals and ceramics with unbroken interfaces. Not only can the instant invention achieve this goal but it can do so in an economical, industrially-scalable manner.